1. Field of the Invention
This invention relates to a vacuum lifting apparatus particularly suitable for lifting large heavy objects having lifting surface deviations caused by the flexibility of the object, manufacturing defects and the like.
2. Description of the Prior Art
Devices for lifting large objects through the development of a suitable vacuum have been generally known because of the substantial nature of the gripping forces which may be developed through the evacuation of a space between the lifting mechanism and the object to be lifted. Vacuum lifting devices have been used quite extensively for lifting large, heavy objects since the problems of lifting large concrete slabs or heavy steel plates are quite readily overcome through the application of a vacuum lifting device.
My U.S. Pat. No. 3,117,815 dated Jan. 14, 1964 relates to a Vacuum Lifter having a rigid structure capable of supporting extremely heavy objects. The lifter utilizes the concept of evacuating the air from an enclosed space between a lifting frame and an object to be lifted, wherein the enclosed space is subdivided by deformable gaskets into inner and outer chambers. The chambers communicate through passages such that total evacuation of the space therebetween is accomplished in several stages. This arrangement makes it possible to engage and disengage the object in a rapid fashion. My commonly assigned U.S. Pat. No. 3,833,251 dated Sept. 3, 1974 relates to a vacuum lifter specifically adapted for lifting arcuately shaped objects by an arrangement of structural members adapted to accommodate arcuately configured objects dimensioned within a suitable range of diameters.
While such devices have proven to be extremely successful in lifting extremely heavy, rigid objects having flat lifting surfaces, as well as relatively lighter curved or arcuate objects, it has been extremely difficult to devise a vacuum lifting apparatus capable of lifting either relatively flexible objects of substantial weight, or large heavy objects having deviations in the planar lifting surface, primarily because the atmospheric grip developed between the frame and the object has been found to be vulnerable to atmospheric leaks, which render the apparatus incapable of supporting the object. For the case of inherent defects in the lifting surface the substantial weight of the object makes it difficult to maintain the atmospheric grip thus developed. For the case of flexible objects, deviations are developed in the lifting surface during lifting when the object flexes.
For flexible objects, the problem is most readily recognized upon consideration of the difficulties inherent in lifting a large steel plate having an area of, say 150-200 square feet and a thickness of 1/8-174 inch. The flexibility of such a plate is such that upon lifting the plate in the central portion with a vacuum lifting device of the known type, the forces of gravity acting downwardly on the outer portions of the steel plate, combines with the upward force provided by the vacuum lifting apparatus and associated lifting devices and results in the development of a compound curvature in the flexible plate. When the curvature of the plate reaches a predetermined level, it cannot be accommodated merely by non-uniform decompression of the gasket, with the result that an air leak is developed and the vacuum is lost, causing the plate to be released in midair.
When the surface of the object to be gripped deviates somewhat from a planar surface, the atmospheric vise thus created by a rigid vacuum lifting device of the type described will also be vulnerable to atmospheric leaks.
Because large objects such as steel plates and the like are often substantial in weight, it is desirable to provide a lifting apparatus having a structure which is sufficiently rigid and capable of supporting such heavy objects while providing sufficient flexibility with respect to the creation of an atmospheric grip so as to accommodate any deviations which may appear in the object lifting surface without affecting adversely the ability to support such heavy objects. I have invented a vacuum lifting device which is not only capable of supporting extremely heavy, rigid, and relatively flexible objects, but which will readily accommodate deviations from a planar character, of the lifting surface, notwithstanding the cause for such deviation.